In the current RO1 AR54604, we are studying the interaction between skeletal acquisition and bone marrow adiposity, and testing the hypothesis that there are shared skeletal and metabolic networks controlled by a series of genes situated on mouse chromosome 6. This competitive revision expands our ongoing work by building on the recent identification by our collaborator, Jorge Plutzky, that the retinoic acid (RA) precursor, retinaldehyde (Rald), functions as a novel biologically active mediator in fat where it determines metabolic phenotypes by repressing PPAR3. The Rosen/Horowitz laboratories have been characterizing congenic (e.g. B6.C3H-6T) and inbred strains of mice (C57BL6 and C3H/HeJ) and identifying genetic determinants of bone mass and IGF-I. We found genes in the mid region of mouse Chr6 that function together as a regulatory unit to determine a skeletal phenotype in 6T mice of low bone mass, impaired osteoblast (OB) differentiation, low circulating IGF-I, and enhanced bone marrow adiposity. Although 6T mice lose bone after a high fat diet (HFD) challenge, their metabolic profile is characterized by protection from diet-induced insulin resistance and obesity. Remarkably, the location of this small genomic cluster (4Mb) on mouse Chr 6 includes 4 genes important in adipogenesis: peroxisome proliferator-activated receptor-gamma (PPAR3), Alox5, SDF-1, and FSP 27. Recently we identified several polymorphisms within the 3'UTR of the PPAR3 gene, and examined the downstream effects of this 'gain of function'on targets such as IGF-I. These data provided an important rationale for studying the mechanisms of adipogenic differentiation in marrow stromal cells. After the Plutzky group reported that Rald, which previously had no documented role outside of the retina, inhibited PPAR3-RXR and repressed adipogenesis in vivo, we began a collaboration to determine Rald's mechanism of action in bone. We analyzed skeletal phenotypes from retinaldehyde dehydrogenase null (Raldh1-/-) mice and found markedly increased femoral areal bone mineral density in 12 week Raldh1-/- mice vs B6. In addition, there was more than twice the femoral bone volume fraction in the null mice compared to controls. These lines of evidence now permit us to test how Rald, either through the RXR- PPAR complex, or independently, has a critical regulatory role in stromal cell fate, and ultimately in bone acquisition. We will accomplish this by fully characterizing the skeletal phenotype of the Raldh1-/- mice on regular and high fat diets as well as with and without rosiglitazone. We will interrogate the molecular mechanisms in osteoblasts and osteoclasts to determine the presence of Rald and Raldh1 and the effects of the retinoid axis on bone marrow stromal cell allocation and osteoclastogenesis. PUBLIC HEALTH RELEVANCE: The importance of nuclear receptor networks in bone marrow differentiation programs has been firmly established and in our ongoing grant, AR54604, we identified several genetic polymorphisms in the PPAR3 gene which we identified as a key determinant of adipocyte and bone cell fate. We now propose to study retinaldehyde and its effect on RXR which partners with PPAR3, to form an activated transcriptional complex to regulate gene expression and ultimately to influence bone mass acquisition. A more comprehensive understanding of the regulatory inputs that determine PPAR3 responses in bone has important clinical implications, particularly after the discovery that the thiazolidinediones, drugs that enhance insulin sensitivity and are exogenous ligands for PPAR3, cause bone loss and fractures.